order of magnitude

Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

n. An estimate of size or magnitude expressed as a power of ten: Earth's mass is of the order of magnitude of 1022 tons; that of the sun is 1027 tons.

n. A range of values between a designated lower value and an upper value ten times as large: The masses of Earth and the sun differ by five orders of magnitude.

from Wiktionary, Creative Commons Attribution/Share-Alike License

n. The class of scale or magnitude of any amount, where each class contains values of a fixed ratio (most often 10) to the class preceding it. For example, something that is 2 orders of magnitude larger is 100 times larger, something that is 3 orders of magnitude larger is 1000 times larger, and something that is 6 orders of magnitude larger is a million times larger, because = 100, = 1000, and = a million.

n. a number assigned to the ratio of two quantities; two quantities are of the same order of magnitude if one is less than 10 times as large as the other; the number of magnitudes that the quantities differ is specified to within a power of 10